Low-emissivity coatings are well known in the present art. Typically, they include one or more infrared-reflective layers each positioned between two or more dielectric layers. The infrared-reflective layers reduce the transmission of radiant heat through the coating. The infrared-reflective layers typically comprise electrically-conductive metals, such as silver, gold, or copper. The dielectric layers reduce the visible reflectance of the coating and control other coating properties, such as color. Commonly used dielectrics include oxides of zinc, tin, and titanium, as well as nitrides, such as silicon nitride.
Manufacturers have historically provided a single, thick dielectric layer on each side of each infrared-reflective layer. Reference is made to U.S. Pat. No. 4,859,532, the entire contents of which are incorporated herein by reference. Thick dielectric layers, however, are less than ideal in several respects. For example, the stress in a dielectric layer increases with increasing layer thickness. This is particularly problematic with dielectric films that inherently have high stress, such as silicon nitride. Further, it has been discovered that haze formation is likely to occur in heat-treatable (e.g., temperable) coatings that comprise thick dielectric layers. U.S. patent application Ser. No. 09/728,435, entitled Haze-Resistant Transparent Film Stacks, the entire contents of which are incorporated herein by reference, addresses this problem and replaces thick dielectric layers with a plurality of thin dielectric layers.
Typically, the dielectric layers in a low-emissivity coating are homogenous. That is, each dielectric layer typically has a composition that is uniform over the thickness of the layer. While homogenous dielectric layers have gained widespread acceptance, they have significant limitations. For example, the adhesion properties are limited for a low-emissivity coating wherein all the dielectric layers are homogenous. This is due in part to the discrete interfaces that exist between homogenous dielectric layers. Stress tends to pile up (i.e., be concentrated) at each discrete interface in a low-emissivity coating. Therefore, each such interface is a potential delamination site that is preferably avoided.
Further, the optical opportunities are limited for a low-emissivity coating wherein all the dielectric layers are homogenous. A coating of this nature may only achieve limited color and antireflection properties due to the optical limitations of having each dielectric layer in the coating be homogenous.
As noted above, the primary optical function of the dielectric films in a low-emissivity coating is to antireflect the infrared-reflective film (e.g., silver) in the coating. The dielectric films, however, desirably provide additional functions. Consider a double-silver coating comprising a dielectric inner coat (between the substrate and the first silver layer), a dielectric middle coat (between the two silver layers), and a dielectric outer coat (further from the substrate than the second silver layer). Each of these coats preferably has specific characteristics, as do the inner and outer interfaces of each coat.
Insofar as the dielectric inner coat is concerned, the inner interface of this coat preferably provides good adhesion to the substrate. It is desirable to assure the base coat adheres well to the substrate, as this coat serves as the foundation for the coating. In some cases, it is also desirable that the outer interface of the inner coat provide good growth conditions for silver film. The electrical conductivity (and hence the emissivity) of a silver film varies depending upon the particular surface on which the silver is deposited. Thus, when a silver film is provided directly over the dielectric inner coat, the inner coat desirably has an outer interface that provides a good nucleation surface on which to grow silver film. In such cases, this outer interface preferably also adheres well to the overlying silver film. Further, the outer interface in such cases preferably immobilizes the overlying silver as much as possible (particularly during heat treatment). It is to be appreciated that in some cases a metal blocker film or another non-dielectric film is alternatively placed beneath a silver film to achieved desired durability and/or optical and/or insulating properties. The dielectric inner coat preferably prevents sodium ions and other material from diffusing out of a glass substrate (i.e., it preferably seals the glass). This is desirable to protect the first silver layer against being corroded from below.
Unfortunately, it is difficult to optimize all these properties using an inner coat formed by a single layer of any one material. As an alternative, the inner coat can be formed of two or more discrete layers of different materials, each chosen to optimize one or more of the desired coating properties. However, this leaves the inner coat with an additional interface which, as noted above, is preferably avoided.
The situation is similar for the dielectric outer coat. For example, the outer coat preferably defines an inner interface that adheres well to the underlying film (e.g., to the second silver layer or the second blocker layer). The outer coat desirably contributes to the mechanical and chemical durability of the coating. For example, the outer coat preferably comprises a chemically durable material. Conjointly, the outer coat preferably defines a smooth outer surface, so as to reduce the coating's vulnerability to being physically abraded. Finally, the outer coat preferably comprises film that prevents moisture, oxygen, and other reactive agents from diffusing to the underlying silver (particularly during heat treatment and over time). This is desirable to protect the second silver layer against being corroded from above. As with the inner coat, it is difficult to optimize all the desired properties with an outer coat formed by a single layer of one material, yet forming the outer coat of two or more discrete layers of different materials yields an additional interface, which is preferably avoided.
With respect to the dielectric middle coat, it is particularly desirable to optimize the properties and functions of the dielectric film used in this coat. This is due in part to the great thickness of the middle coat. (The middle coat is characteristically thicker than the inner and outer coats.) It is particularly desirable, for example, to minimize the stress in the middle coat. This is preferably accomplished by limiting the thickness of each layer in the middle coat. As noted above, the stress in a dielectric layer tends to increase with increasing layer thickness. Thus, by limiting the thickness of each layer in the middle coat (or at least those layers comprising high stress material), stress can be advantageously reduced.
It is also desirable to provide a middle coat that prevents defects from growing over the entire thickness of the middle coat. This can be accomplished by providing a middle coat that comprises a plurality of dielectric layers. In such a middle coat, defects (e.g., pinholes and the like) are less likely to propagate from one layer to another, especially when contiguous layers are formed of different materials. Thus, by providing a middle coat comprising a plurality of dielectric layers, it is less likely that defects will grow across the entire thickness of the middle coat.
Further, it is advantageous to provide a middle coat that is resistant to the haze formation that can occur, e.g., during heat treatment. This can be accomplished by providing a middle coat comprising a plurality of particularly thin dielectric layers, preferably formed of particular materials. While this solution has great benefit, it is less than ideal in that it creates additional interfaces in the middle coat.
Still further, the middle coat preferably defines an inner interface that adheres well to the underlying film (e.g., to the first silver layer or the first blocker layer). Conjointly, in cases where silver is positioned directly over the middle coat, the outer interface of the middle coat preferably provides good growth conditions for the overlying silver layer. In such cases, this outer interface preferably adheres well to the overlying silver film and immobilizes the overlying silver film as much as possible.
It is extremely difficult to optimize all these properties using a middle coat formed by a single layer of any one material. Thus, the middle coat can alternatively be formed by a plurality of discrete layers of different dielectrics, each chosen to optimize one or more properties. This, however, is less than ideal in that it leaves the middle coat with additional interfaces, which are preferably avoided.
It would be desirable to provide a low-emissivity coating that minimizes the foregoing limitations and optimizes the foregoing properties and functions.